The present application is based on and claims priority from Japanese Patent Applications: Hei 11-300442, filed Oct. 22, 1999, Hei 11-300480, filed Oct. 22, 1999, Hei 11-300492, filed Oct. 22, 1999, and 2000-45589, filed Feb. 23, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a shock absorber for absorbing a shock.
2. Description of the Related Art
JP-A-63-277859 discloses a starter equipped with a shock absorber for interrupting transmission of an excessive torque. In such a shock absorber for a starter, a rotary disk is pressed against a stationary disk to generate a frictional force so that rotation of the rotary disk can be controlled. Accordingly, when a shock that is more than a certain normal torque is applied to the rotary disk, the rotary disk slips or rotates, thereby absorbing the shock.
Such a rotary disk has a plurality of dimples and small banks formed around the dimples at a height of about tens of microns, providing a press-formed frictional surface. An amount of grease is filled in the spaces (dimples) between the stationary disk and the rotary disk with the banks being in contact with the surface of the stationary disk to prevent seizing.
Because an array direction of the dimples is the same as the direction of the rolling of the raw material of the rotary disk, the height of the banks are not formed even. That is, the bank of the sides in parallel with the direction of rolling is lower than the bank of the sides that is perpendicular to the direction of rolling. Only two sides of the rectangular dimples can touch the stationary disk. Therefore, the two side-banks may be worn away more easily than the four side-banks. This lowers the lifetime of the shock absorber.
If another two-side banks start touching the stationary disk after the first two side-banks initially touched the stationary disk have worn away, the friction factor changes too abruptly to control the transmission torque.
As the rotary disk has rotated for a long time, the spaces for the grease between the head of the dimple and the bottom thereof become narrower. Accordingly, a sufficient amount of grease can not be supplied to the sliding surfaces of the disks. This causes the seizing and shortens the lifetime.
The invention has been made in view of the above problem and has an object of providing a shock absorber that can make the height of the banks (or raised portions) of the rectangular dimples even.
According to a main feature of the invention, rotary disk of a shock absorber has a plurality of press-formed rectangular dimples at a surface opposite the stationary disk. The rectangular dimples have four sides that incline to a direction of rolling of the raw material of the rotary disk. In such a case, the difference in height between the four side banks can be effectively reduced so that four banks can touch the stationary disk evenly.
Preferably, each the rectangular dimples may be square dimples. In such a case, each side of the square shape inclines to the direction of rolling at 45 degree in angle.
The plurality of rectangular dimples can be formed on the surface of the stationary disk instead of the rotary disk.
According to another feature of the invention, a rotary disk of the shock absorber has an oil hole or an oil groove as an oil reservoir. The oil hole or oil groove penetrates the rotary disk in the thickness direction.
Even if the surface of the rotary disk wears away to some extent, the oil hole or oil groove does not disappear, so that grease can be prevented from reducing. The grease filled in the oil hole or oil groove is supplied to the sliding surfaces as long as the rotary disk rotates, so that the seizing can be prevented.
Preferably, the rotary disk has a plurality of oil holes at circumferentially and radially different positions, and the plurality of oil holes are disposed so that the radial positions thereof partially overlap each other on a circumference of the rotary disk. When the rotary disk rotates, a plurality of rotation loci of the oil holes is formed to partly overlap each other.
Accordingly, the supply of the grease is not stopped in the radial direction of the rotary disk where the plurality of the oil holes are formed, so that the grease can be continuously and uniformly supplied to the friction surface of the rotary disk.
The plurality of oil holes is preferably disposed spiral about the center of the rotary disk over the circumference of the rotary disk. The plurality of oil holes is dispersed evenly in the circumferential and radial directions so that the flatness of the rotary disk""s surface opposite the stationary disk can be secured. This prevents uneven contacts with the stationary disk.
The rotary disk can have a first one of the oil holes that opens radially outward and a second one of the oil holes that opens radially inward, and the first one and the second one of the oil holes extend to partially overlap each other in the radial direction. Accordingly, the first one and the second one of the oil holes partially overlap each other when the rotary disk rotates. Therefore, the grease can be supplied evenly to the friction surface of the rotary disk.
In addition, the rigidity of the rotary disk becomes smaller than the oil holes not open to either outward or inward, so that the rotary disk can deform along the stationary disk. This equalizes the pressure on the surface of the rotary disk to result in even frictional wear, so that the lifetime of the rotary disk can be increased.
The first one and second one of the oil holes preferably incline to the rotational direction from radial directions, so that the grease can enter the first and the second ones of the oil holes while the rotary disk rotates. In other words, the grease can be continuously supplied to the friction surface of the rotary disk.
The stationary disk of the shock absorber can have oil reservoirs instead of the rotary disk.
In this case, the same effect as the rotary disk having the oil reservoir can be obtained. Therefore, reduction in strength of the rotary disk can be avoided.
According to another feature of the invention, a rotary disk of a shock absorber is comprised of a plurality of circumferentially divided pieces.
Because the surface area of each the divided piece is small relative to the entire surface area of the rotary disk, each divided piece can have a high flatness. As a result, pressure can be applied to each divided piece evenly, so that the surface pressure of the rotary disk can be made even.
A stationary disk can have a plurality of circumferentially divided pieces, instead of the rotary disk.